1. Field of the Invention
The present invention relates to a tank assembly and more particularly to a slurry tank into which patterns are dipped in the formation of shell molds.
2. Description of the Prior Art
In the making of shell molds for investment castings, particularly by processes in which the shell consists of many layers formed by alternately dipping the pattern in a slurry and dusting with a granulate mixture, various characteristics and conditions of the slurry must be monitored to control the quality of the shell. For example, the slurry must be mixed continuously to prevent its heavier elements from settling. The typical dip tank in present foundry service is essentially an open drum mounted on a vertical shaft which is rotatable. A scraper paddle is positioned internal of the drum adjacent to the vertical wall and extends along the bottom of the drum. As the drum is rotated, the paddle provides continual mixing of the contained material and also avoids the buildup of an excessively thick layer along either the wall or bottom of the drum. As a practical matter, the consistency of a slurry maintained in such a device is not always as uniform as desired. Another consideration affecting control of the quality of molds made with such equipment is the buildup of slurry on the inside surfaces of the rotating drum. Slurry material adheres to the drum and forms a coating having a thickness equal to the clearance between the scraper bar and the drum. Any additional buildup is removed from the coating by the scraper paddle by the mixing action producing hardened particles which find their way into the final shell mold thereby weakening the mold and causing imperfections on the surface of the casting.
Another significant consideration in the quality control of shell molds is the viscosity of the slurry. If the viscosity is allowed to deviate from a predetermined range of values, the thickness of the shell mold formed in each dip process can become either too thick or too thin. At present, the viscosity is measured with a manual technique using a Zaun cup in which an operator fills the cup with the slurry mixture to be tested and observes the time required for the Zaun cup to empty. This time is correlated to viscosity. The technique is subject to human judgment and can be inaccurate.
Control of the temperature of the slurry throughout the dipping process is also important. The temperature of the slurry itself is usually not critical, however, the temperature to which the pattern is exposed can be very critical. If the temperature of a wax pattern is allowed to vary to any substantial extent, the dimensional control of the cast item can be compromised. Thus, by maintaining the temperature of the slurry within a prescribed range this source of dimensional error can be eliminated.
Another significant variable in the slurry composition is density. In a slurry dip operation using rotating drum apparatus, the slurry density is typically controlled manually with the Zaun cup apparatus mentioned previously; both viscosity and density can be measured simultaneously. One of the principal shortcomings of this technique is the inaccuracy in the observed readings which require a human operator. In some processes such as the preparation of shell molds for fine investment castings, density control by Zaun cup measurement is not sufficiently accurate. Also since this control is done manually, the tendency is to take sample readings at rather lengthy intervals and adverse changes in the density characteristic can occur during such intervals, resulting in poor quality molds. The effect on a mold made from a slurry having an improper density can be considerable. For example, since the smaller mesh particulate in a slurry tends to be consumed before the large mesh material, the density of the slurry tends to increase with usage. This type of change can be adverse to the surface quality of the cast part. Also, an increase in the average mesh size of the particles forming the slurry has been found to decrease the strength of the mold.